Relations with find_each

I’m a bit unclear what’s going on with relations. I’m hoping someone can help explain it to me.

Lets say we have a model with orders and line_items such that an order has many line_items. Thus:

class Order

has_may :line_items

end

class LineItem

belongs_to :order

end

When I have an order and call line_items what’s going on? e.g.

Order order = Order.first

order.line_items.each {|li| puts li }

I thought that was basically an alias for:

Order order = Order.first

LineItem.where(:order_id => order.id).each {|li| puts li }

but, this isn’t consistent with batching.

So if we do

Order order = Order.first

this doesn’t appear to work, it’s not batching, just finding all of the line items and iterating

order.line_items.find_each {|li| puts li }

this seems to work as expected (i.e. it batches the retrieval from the db)

LineItem.where(:order_id => order.id).find_each {|li| puts li }

Can anyone explain to me why these to are different?

John H. wrote in post #1063205:

I'm a bit unclear what's going on with relations. I'm hoping someone can
help explain it to me.

Lets say we have a model with orders and line_items such that an order
has
many line_items. Thus:

class Order
  has_may :line_items
end
class LineItem
  belongs_to :order
end

When I have an order and call line_items what's going on? e.g.

Order order = Order.first
order.line_items.each {|li| puts li }

I thought that was basically an alias for:

Order order = Order.first
LineItem.where(:order_id => order.id).each {|li| puts li }

This is an over-simplification. Let me illustrate by example:

1.9.3p194 :010 > order = Order.first
  Order Load (0.3ms) SELECT "orders".* FROM "orders" LIMIT 1
=> #<Order id: 1, name: "First item", description: "First one", price:
nil, created_at: "2012-06-06 01:20:03", updated_at: "2012-06-06
01:20:03">

Notice first that selecting the order does not touch the line_items
table at all. At this point the order.line_items method is represented
by an ActiveRecord::Relation object. Accessing this relation object will
load (fire) the relation and populate the array.

Now let's take look at the line_items:

1.9.3p194 :002 > order.line_items
  LineItem Load (0.2ms) SELECT "line_items".* FROM "line_items" WHERE
"line_items"."order_id" = 1
=> [#<LineItem id: 1, order_id: 1, name: "Line 1", quantity: 5,
created_at: "2012-06-06 01:21:24", updated_at: "2012-06-06 01:21:24">]
1.9.3p194 :003 > order.line_items
=> [#<LineItem id: 1, order_id: 1, name: "Line 1", quantity: 5,
created_at: "2012-06-06 01:21:24", updated_at: "2012-06-06 01:21:24">]

Notice the first time we access line_items ActiveRecord will load the
association by running a generated SQL statement. Subsequent calls do
not reissue the SQL statement.

Let's take a look at what is reported as the class for order.line_items:

1.9.3p194 :003 > puts order.line_items.class
Array

This appears to be a typical Array object, but it's really more
complicated than that. What you're seeing is not a simple Array object,
but rather an Array that has been extended with some Rails magic. Let's
take a look that the ancestry of that resulting "Array" object:

1.9.3p194 :004 > puts order.line_items.class.ancestors
Array
JSON::Ext::Generator::GeneratorMethods::Array
Enumerable
Object
PP::ObjectMixin
JSON::Ext::Generator::GeneratorMethods::Object
ActiveSupport::Dependencies::Loadable
Kernel
BasicObject

Here's the ancestry of a plain old Ruby Array:

# Ruby (No Rails)
$ irb
1.9.3p194 :001 > Array.ancestors
=> [Array, Enumerable, Object, Kernel, BasicObject]

In fact all Array objects in rails have the additional magic added to
them:

# Ruby on Rails
$ rails console
Loading development environment (Rails 3.2.3)
1.9.3p194 :001 > puts Array.ancestors
Array
JSON::Ext::Generator::GeneratorMethods::Array
Enumerable
Object
PP::ObjectMixin
JSON::Ext::Generator::GeneratorMethods::Object
ActiveSupport::Dependencies::Loadable
Kernel
BasicObject

but, this isn't consistent with batching.

So if we do

Order order = Order.first

# this doesn't appear to work, it's not batching, just finding all of
the
line items and iterating
order.line_items.find_each {|li| puts li }

Hopefully what I explained above will clear up what's happening here.
That very first access of order.line_items is causing that relation
object to fire and load the objects.

What you probably want is the find_in_batches method provided by
ActiveRecord:

http://api.rubyonrails.org/classes/ActiveRecord/Batches.html#method-i-find_in_batches

Thanks.

(the find_each I referenced in my question is (basically) the same as find_in_batches)

The question is really why:

order.line_items != LineItem.where(:order_id => order.id)

So, I think what you’re saying is that

order.line_items returns an array, not a relation.

order.line_items != LineItem.where(:order_id => order.id)

Let me rephrase Robert's words.
The key difference here is, that LineItem.where(:order_id => order.id)
doesn't fire the sql and returns ActiveRecord::Relation object.
While the order.line_items runs SQL and returns the Array of records.
The purpose of it is that u can change the resulting SQL and do the
method chaining.

Try next:
line_items_by_order_id = LineItem.where(:order_id => order.id) # =>
object of ActiveRecord::Relation class
ordered_line_items_by_order_id = LineItem.where(:order_id =>
order.id).order(:updated_at) # => object of ActiveRecord::Relation
class
ordered_line_items_by_order_id_with_positive_price =
LineItem.where(:order_id =>
order.id).order(:updated_at).where("line_items.price > ?", 20) # =>
object of ActiveRecord::Relation class
# ... and so on
# where
order.line_items # returns [LineItem, LineItem ...] array

It is noteworthy that one can still do
order.line_items.where( some condition )
even though order.line_items appears to be an Array.

Colin

I think that’s what’s always tripped me up. Most ActiveRecord commands still work normally on the array. I’ve always chained them and thought I was building a relation, not getting the array. It wasn’t until I had a big set of records and discovered I couldn’t use the association in a find_each that I tripped over this.

Thanks for the help.